This study is concerned with the biological and chemical characteristics of the psittacosis organisms, with special emphasis on antigenic, chemical and biological analyses of the cell envelopes of meningopneumonitis agent, a member of this group. Large scale production of the organism at different stages in its developmental cycle will make possible further studies on the following: 1) The characteristics of the carbohydrate, amino sugar and lipid components of the envelope. 2) The isolation and nature of the antigens located in the cell envelopes. 3) The effect of penicillin and other cell envelope synthesis inhibitors on the envelope structure. 4) Enzyme activity of the organism during development.